In numerous etching and coating processes plasmas are generated out of which ions are accelerated onto a substrate. In order to be able to control the thickness of a layer or the depth of etching, a dc voltage is provided between two electrodes which enclose a plasma.
If electrically non-conducting layers are to be deposited on a substrate, instead of a dc voltage an ac voltage must be applied in order to build up a dc potential at the substrate which is required for the acceleration of charged particles, in particular of ions, in the direction toward the substrate.
For the coating or etching effect the generation of a stationary potential difference between plasma and electrode is essential, which in the case of an HF voltage applied at the electrode is achieved thereby that during the positive half period significantly more electrons can stream onto the electrode surface than positive ions can escape from it during the negative half period. But since averaged over time an identical quantity of positive and negative charge carriers are carried out of the plasma, the electrode becomes negatively charged relative to the plasma until almost throughout the entire length of the period positive ions can stream in. Before the electrode are generated positive space charge regions leading to a rectifier effect and to the formation of a barrier layer capacitance (cf. DEP 37 08 717, column 5, line 20 to column 6, line 6).
A large number of coating installations with ac current feeds at electrodes are already known which include a plasma volume (U.S. Pat. Nos. 3,461,054, 4,874,494, 4,719,154, 4,424,101, 3,767,551, 3,617,459, 4,572,842; P. Vratny: Deposition of Tantalum and Tantalum Oxide by Superimposed RF and DC Sputtering, J. Electrochem. Soc., Vol. 114, No. 5, May 1967, p. 506, FIG. 1; EP-A-O 347 567; K. Kohler, J. W. Coburn, D. E. Horne, E. Kay: Plasma potentials of 13.56 MHz rf argon glow discharges in a planar system, J. Appl. Phys. 51 (1), January 1985, pp. 59-66; Butler and Kino: Plasma Sheath Formation by Radio-Frequency Fields, The Physics of Fluids, Vol. 6, No. 9, Sept. 1963, pp. 1346-1355; A. J. van Roosmalen, W. G. M. van den Hoek and H. Kalter: Electrical properties of planar rf discharges for dry etching. J. Appl. Phys. 58, July 1985, pp. 653-658; EP 0 470 580. These installations, however, are not suitable for the so-called in-line operation in which several substrates are successively carried past an electrode.
Furthermore is known an arrangement in which substrates to be coated are moved within a receptacle (DE-OS 29 09 804). The high-frequency voltage for the generation of a plasma is herein applied via lines to a substrate holder and to a target holder. Details about the disposition and mounting of the substrate are not shown.
Furthermore is known a plasma reactor in which a first and a second electrode are disposed one opposing the other and wherein a third electrode is provided between the first and the second electrode (EP-A-0 139 835). The first electrode herein is at ground potential while the second electrode is connected to an ac voltage of approximately 100 KHz and the third electrode is supplied with an ac voltage of 13.56 MHz. The electrode disposed between the two other electrodes is herein shaped annularly. However, an in-line operation is also not possible with this plasma reactor because the substrate to be coated is stationarily disposed on the second electrode.
Moreover, an arrangement for etching substrates through a glow discharge is known in which the electrode opposing a substrate carrier is provided in the margin zone with a projection at the same potential, which bridges the volume between the electrode and the substrate or the substrate carrier, with the exception of a gap of approximately 5 mm, in such a way that the glow discharge is limitable to the volume between substrate carrier and electrode (DEP 22 41 229). Because of a closed bell over the substrate carrier and because of the stationary disposition of this substrate carrier this known arrangement is not suitable for an in-line installation. In addition, an HF substrate bias voltage is not settable.
In order to avoid the undesirable effects due to glow discharges, it has been suggested for another known arrangement to minimize the ratio of the area of the electrode supplied with voltage to the area of all other surfaces in contact with the discharge and to use as large a vacuum chamber as possible so that the edges of the electrode are disposed far enough from the chamber walls (J. L. Vossen, Glow Discharge Phenomena in Plasma Etching and Plasma Deposition, J. Electrochem. Soc. SSST, Vol. 126, 1979, pp. 391-324). But any possibility for realizing an in-line installation is also not shown in this arrangement.
In order to be able to apply in the dynamic coating process in in-line systems, in which the substrates are carried by a transport system during the coating past the coating source, the high-frequency voltage to the electrodes it would be conceivable to couple them galvanically via a sliding or roller contact. However, herein parasitic plasmas would occur in the region of the voltage coupling, which could only be suppressed through a technically expensive dark space shield of the transport system. Moreover, the sliding or roller contacts would in time become coated, which would have a negative effect on the power transfer especially if the coating comprises an electric insulator. Added to this is the fact that the strong mechanical strain occurring when using sliding contacts or contact rollers would support the generation of particles which, in turn, would reduce the quality of the layer.